Clarify zh-CN MCP registry fallback wording

[w287346141]

Summary

• clarify the zh-CN MCP registry source description by naming Smithery as a third-party directory
• replace the mixed Chinese/English fallback wording with 本地备选
• add i18n regression coverage for the zh-CN wording

Verification

• npm.cmd run test -- slash-help-i18n.test.ts
• npm.cmd run typecheck
• npm.cmd run verify
• manual: Select-String confirms Smithery 第三方目录 exists and 本地 fallback has no matches